The water absorbent resin has been hitherto used as one component for hygienic materials such as sanitary cotton, disposable diaper, and absorbents for other kinds of body fluid. As concrete examples of the water absorbent resin, hydrolyzate of starch-acrylonitrile graft polymer, neutralized starch-acrylic acid graft polymer, saponified vinyl acetate-acrylic acid ester copolymer, hydrolyzate of acrylonitrile copolymer or acrylamide copolymer, and the product of crosslinkage thereof, and partially neutralized crosslinked acrylic acid may be cited. These water absorbent resins invariably possess an internal crosslinked structure and exhibit no solubility in water.
The characteristic properties which these water absorbent resins are expected to possess include high absorption capacity, perfect absorption speed, high gel strength, and fully satisfactory suction force necessary for sucking water from a medium, for example. Since the water absorbing properties are affected by crosslink density, they do not necessarily manifest positive correlations with one another as evinced by the fact that an increase in the crosslink density leads to an increase in the gel strength but a decrease in the amount of water absorbed. Particularly, the absorption capacity is in a contradictory relation with the absorption speed, the gel strength, and the suction force, for example. The water absorbent resin which has acquired an enhanced absorption capacity, therefore, possibly shuns uniform absorption of water and forms portions of partial aggregation of itself when the water absorbent resin particles contact with water and induces extreme deterioration of the absorption speed because the water is not diffused throughout the entire volumes of water absorbent resin particles.
For the purpose of relaxing this phenomenon and obtaining a water absorbent resin which has a high absorption capacity and a comparatively satisfactory absorption speed, a method for giving the water absorbent resin particles a surface coated with a surfactant or a nonvolatile hydrocarbon has been available. This method indeed exalts the dispersibility of the initially absorbed water but brings no sufficient effects in enhancing the absorption speed and the suction force of the individual resin particles.
As a means to produce a polyacrylic acid type polymer of high water absorbing property, a method which comprises causing an aqueous composition having a partial alkali metal salt of polyacrylic acid as a main component and having a low crosslink density to be heated in the presence of a water-soluble peroxide radical initiating agent thereby introducing a crosslink therein by radical crosslinkage has been proposed (U.S. Pat. No. 4,910,250). It is difficult to distribute uniformly internal crosslinks in the polymer and uneasy to adjust the crosslink density. Thus, a measure of preparing a polymer which contains water-soluble polyacrylic acid gel having low crosslink density and then heating the polymer together with a persulfate added thereto as a polymerization initiator is adopted. U.S. Pat. No. 4,910,250 claims to realize precise control of crosslink density by adjusting the amount of the initiating agent to be added and, owing to the uniform presence of crosslink in the polymer, acquire perfect water absorbing properties and obtain as well a water absorbent resin devoid of stickiness.
While the persulfate which is used in U.S. Pat. No. 4,910,250 mentioned above is decomposed by heat, it is decomposed by ultraviolet rays and generates radicals (J. Phys. Chem., 1975, 79, 2693, J. Photochem. Photobiol., A. 1988, 44, 243). Since the persulfate fulfills a function as a polymerization initiator, the irradiation of the aqueous solution of a water soluble vinyl monomer with a light energy induces the initiator to dissociate and form a radical and enables the resultant monomer radical to attack the next monomer and produce a synthetic polymer gel (JP-A 2004-99789). The method of JP-A 2004-99789 is supposed by irradiating an aqueous solution containing a water-soluble vinyl monomer of a specific structure and a persulfate with ultraviolet rays to effect the polymerization of the water-soluble vinyl monomer and the cross-linkage of the formed polymer simultaneously. Incidentally, a reaction system which forms an internal crosslink by adding a hydrophilic polymer component, a photo-polymerization initiator, and a crosslinking agent together and irradiating them with ultraviolet rays has been known (WO 2004/031253). By the method in WO 2004/031253, a cross-linked hydrogel is obtained by using a persulfate as a photopolymerization initiator and irradiating the aqueous solution of a water-soluble polymer with ultraviolet rays in the presence of a cross-linking agent.
Meanwhile, a method which gives a water absorbent resin a surface treatment with a crosslinking agent and imparts thereto a surface of a heightened crosslink density has been also known (U.S. Pat. No. 4,666,983 and U.S. Pat. No. 5,422,405, for example). The water absorbent resin, as described above, is a water-insoluble polymer having an internal crosslink which is produced by incorporating an inner crosslinking agent and a polymerization initiator in a polymerizing monomer and polymerizing the monomer. On the surface of the water absorbent resin resulting from the polymerization, the reactive functional groups contained in the monomer are present. By introducing cross-linkage between the functional groups by adding a surface crosslinking gent capable of reacting with such functional groups, therefore, the resultant water absorbent resin has improved crosslink density and can manifest excellent water absorption properties even under pressure.
Since the use of the surface crosslinking agent mentioned above requires the reaction for the formation of crosslinks to be performed at a high temperature for a long time and entails the problem of suffering persistence of the crosslinking agent in the unaltered state, a method which, by causing an aqueous solution containing a peroxide radical initiating agent to contact with a resin and heating the resin, accomplishes introduction of crosslinks into polymer molecular chains in the neighborhood of the surface of the resin by virtue of decomposition of the radical initiating agent has been proposed (U.S. Pat. No. 4,783,510). In a working example of this method, a water absorbent resin exhibiting an exalted absorption capacity was obtained by effecting the heating with superheated steam at 130° C. for 6 minutes. Further, a water absorbent resin manifesting an exalted water absorption capacity was obtained by incorporating further such a hydrophilic polyfunctional unsaturated compound as N,N′-methylenebisacrylamide in an aqueous solution containing a peroxide radical initiator, allowing the aqueous solution to contact with a resin, and thereafter heating it.
A method which comprising impregnating a water absorbent resin with a water-soluble ethylenically unsaturated monomer, polymerizing the resin, and heating the resultant polymer, to produce a water absorbent resin of enhanced quality possessing a higher cross-link density in the neighborhood of the surface of resin particles than in the interior of the resin particles has been also known (Japanese Patent No. 2530668). This water absorbent resin has a higher cross-link density in the neighborhood of the surface than in the interior of the resin particles, because the aqueous solution containing the water-soluble ethylenically unsaturated monomer, during the surface cross-linkage of the water absorbent resin, is caused to contain a crosslinking agent of a higher concentration than the crosslinking agent used during the polymerization of the water absorbent resin and the polymer layer is formed on the surface of the water absorbent resin by using this solution. Japanese Patent No. 2530668 has stated that such a water-soluble radically polymerization initiator as potassium persulfate and sodium persulfate can be used as the polymerization initiator during surface crosslinkage. In one working example cited therein, the polymerization was initiated by heating this initiator to a temperature of 60° C.